1. Field of the Invention
The present invention relates to an image forming apparatus using an electronic photographic method, an electrostatic recording method or the like.
More preferably, the present invention relates to an image forming apparatus forming and carrying a toner image corresponding to an aimed image information on an image carrying material such as an electronic photographic photosensitive material, an electrostatic recording dielectric material or the like in accordance with a suitable image forming process such as an electronic photographic process, an electrostatic recording process or the like, and provided with a transfer member for transferring the toner image to a transfer material and a contact heating type fixing apparatus for making the toner image on the transfer material a permanent fixing image.
2. Related Background Art
Conventionally, an image forming apparatus using an electronic photographic method such as a laser beam printer or the like receives a command relating to a printing, a coded letter and an image information as a data by means of an external information processing device such as a computer or the like, and an image having a concentration information such as a photography or the like is performed a known image processing such as an error diffusion method or the like and is converted into an image information made binary.
Next, the image information is printed in an electronic photographic engine portion. FIG. 12 shows a schematically cross sectional view of the electronic photographic engine portion in accordance with the conventional art.
The electronic photographic engine portion is structured such that a primary charging unit 2 for charging a photosensitive drum (a photosensitive material) 1 as an image carrying material along a rotational direction thereof, exposing means 3 for exposing the photosensitive drum 1 and forming an electrostatic latent image, a developing device 4 for attaching a toner (a developer) to the electrostatic latent image and forming a toner image, a transfer roller (a transfer device) 5 for transferring the toner image on the photosensitive drum 1 to a transfer material P and a cleaning device 7 for removing a residual toner are arranged in a periphery of the photosensitive drum 1. The transfer material P corresponding to a transfer target of the toner image is supplied and conveyed from a sheet cassette (not shown) to the photosensitive drum 1. The transfer material P supplied to the photosensitive drum 1 is transferred the toner image by the transfer roller (the transfer device) 5, and thereafter, conveyed to a fixing device 6, and here, the transfer material P on which the toner image is fixed is discharged out of the apparatus.
Next, the roller transfer device 5 will be described below.
The transfer roller device 5 is structured such as to absorb and transfer the toner image in the side of the photosensitive drum 1 to a surface side of the transfer material P in an electrostatic manner by applying a voltage having a reversed polarity against the toner to the transfer roller 5 while the transfer material P is held between and conveyed by an electric conductive elastic roller (the transfer roller 5) and the photosensitive drum 1.
As a method of controlling a transfer voltage, in order to prevent a poor transfer and a track of a paper on the photosensitive drum 1 caused by a change in resistance of the transfer roller 5 or the like, there has been known an active transfer voltage control (ATVC) for estimating a resistance of the transfer roller and suitably controlling a transfer voltage as disclosed in Japanese Patent Application Laid-Open No. 2-123385.
Since the ATVC is structured such as to flow a desired constant current from the transfer roller to the photosensitive drum during a preliminary rotating process of the image forming apparatus, estimate a resistance of the transfer roller 5 on the basis of a bias voltage at that time and apply a constant voltage bias corresponding to the resistance to the transfer roller 5 as a transfer bias at a time of transferring in the printing process, a poor transfer, a track of a paper and the like can be prevented.
On the contrary, as a heating and fixing device for fixing the toner image transferred on the transfer material as a permanent fixing image, a device of a thermal roller type or a film heating type is widely used. Particularly, as a film heating type, there is a method in which an electric power is not supplied to the heating and fixing device at a time of stand-by and a consumptive electric power is restricted as little as possible. Specifically, a heating and fixing method in accordance with a film heating type of fixing a toner image on a recording material between a heater portion and a pressing roller through a film is suggested in Japanese Patent Application Laid-Open Nos. 63-313182, 2-157878, 4-44075 and 4-204980. FIG. 13 is a schematic view of a main portion of the apparatus. Accordingly, in FIG. 13, the apparatus has a heating member (a heating material, hereinafter refer to a heater) 101 fixed and supported to a stay holder (a supporting material) 102, and an elastically pressing roller 110 press-contacted by forming a nip portion (a fixing nip portion) N with a preset or predetermined nip width with holding a heat resisting thin film (hereinafter, refer to a fixing film) 103 in the heater 101.
The heater 101 is heated by turning on electricity and is adjusted to a preset temperature. The fixing film 103 is a web-like member with an end formed in a cylindrical shape, an endless belt shape or a roll winding shape and conveyed in a direction of an arrow a while being brought into close contact with and sliding on a surface of the heater 101 in a fixing nip portion N by drive means (not shown) or a rotating force of the pressing roller 110.
In a state of heating the heater 101 and adjusting to a preset temperature, and conveying and moving the fixing film 103 in a direction of an arrow, when the transfer material (the recording material) P on which a unfixed toner image t as a heated material is formed and carried is introduced between the fixing film 103 in the fixing nip portion N and the pressing roller 110, the transfer material P is brought into close contact with the surface of the fixing film 103 and carried and conveyed in the fixing nip portion N together with the fixing film 103. In this fixing nip portion N, the transfer material P and the toner image t are heated by the heater 101 via the fixing film 103, so that the toner image t on the recording material P is heated and fixed. The recording material portion passing through the fixing nip portion N is pealed from the surface of the fixing film 103 and conveyed.
A ceramic heater is generally used for the heater 101 as the heating member. For example, the ceramic heater is structured such as to form a current-carrying and heat generating resistance layer 101b such as a silver palladium (Ag/Pb), Ta2N or the like on a surface of a ceramic substrate 101a with an electrical insulating characteristic, a good thermal conductivity and a low thermal capacity such as an alumina or the like (a surface in the side opposing to the fixing film 103) along a longitudinal direction of a substrate (a direction perpendicular to the drawing) by means of a screen printing or the like, and cover the heat generating resistance layer forming surface with a thin glass protecting layer 101c. The ceramic heater 101 is structured such that the current-carrying heat generating resistance layer 101b is heated by turning on electricity to the current-carrying heat generating resistance layer 101b and the whole heater quickly heat the ceramic substrate 101a and the glass protecting layer 101c. The temperature increase of the heater 101 is detected by temperature detecting means 104 disposed at the back surface of the heater and fed back to a current-carrying control portion (not shown). The current-carrying control portion controls a current-carrying with respect to the current-carrying heat generating resistance layer 101b in such a manner that a heater temperature detected by the temperature detecting means 104 is maintained at a preset substantially constant temperature (a fixing temperature). Accordingly, the heater 101 is heated and adjusted to a preset fixing temperature.
The fixing film 103 is structured such that in order to effectively give the heat of the heater 101 to the transfer material P as the heated material in the fixing nip portion N, a thick thereof is set to be significantly thin to a level of 20 to 70 .mu.m. The fixing film 103 is constructed by three layers comprising a film base layer, a primer layer and a mold releasing layer, in which a side of the film base layer is a side of the heater and a side of the mold releasing layer is a side of the pressure roller. The film base layer is made of a polyimide, a polyamide, a PEEK (a polyether ether ketone) or the like having a higher insulation performance than the glass protecting layer 101c of the heater 101, and has a heat resisting performance and a high elasticity. Further, a mechanical strength such as a tensile strength and the like of the whole fixing film 103 is maintained by the film base layer. The primer layer is formed by a thin layer having a thickness of about 2 to 6 .mu.m. The mold releasing layer corresponds to a toner offset prevention layer with respect to the fixing film 103, and is formed by coating with a fluorocarbon resin such as a perphloroalkoxide (PFA), a polytetrafluoroethylene (PTFE), an ethylene propylene fluoride (FEP) or the like at a thickness of about 10 .mu.m.
Further, the stay holder 102 is formed by, for example, a heat resisting plastic member, and serves as a conveyance guide for the fixing film 103 as well as holding the heater 101. In the heating apparatus of the film heating type using the thin film 103 for fixing, since a rigidity of the ceramic heater 101 as the heating member is high, the pressing roller 110 having the elastic layer 111 becomes flat at the pressing portion in correspondence to the flat lower surface of the heater 101 press-contacted thereto and forms the fixing nip portion N having a preset width. In accordance with this structure, a heating and fixing in correspondence to the quick start is realized by heating only the fixing nip portion N.
In the heating and fixing device mentioned above, in the case that the transfer materials such as small-sized papers, for example, envelopes or the like are conveyed in an overlap manner, near an end portion of the papers conveyed in an overlap manner as shown in FIG. 4B, there is a case that a gap is generated between the ceramic heater and the pressing roller. In the case that the gap is generated in this manner, a heat of the ceramic heater does not escape to the pressing roller at that portion, so that a high temperature is locally generated. Accordingly, there has been a case that the film guide is melted and the ceramic heater is broken and deteriorated.